1. Field of the Invention
The present invention relates to two-way radios, pagers and cellular telephones and, more particularly, to demodulating voice/data signals from a carrier signal.
2. Background Art
Digital frequency modulation is popular for radio, pager, and cellular telephone (hereinafter collectively referred to as xe2x80x9cradioxe2x80x9d) applications. A number of different demodulation methods for digital frequency modulation have been proposed. Among the proposed methods, arctangent demodulation is favored in radio applications. Arctangent demodulation, however, is computationally expensive. Specifically, in practice, a digital signal processor (DSP) implementation of an arctangent demodulator is utilized to numerically process digital samples of in-phase and quadrature signals of a voice/data signal demodulated from a composite signal received at the arctangent demodulator, where the composite signal includes a carrier signal having the voice/data signal modulated thereon. The output of the DSP is filtered and amplified to produce the voice/data signal.
In order to process each sample of the in-phase signal (I) and the quadrature signal (Q), the DSP implementation of the arctangent demodulator invokes one division operation, one arctangent operation, and one differentiation operation. Contrary to a multiplication operated by the DSP, however, division and arctangent operations are cycle-intensive operations. For example, the DSP implementation of a typical arctangent demodulator utilizes a seventh order polynomial to compute the arctangent. This seventh order polynomial requires six multiplication operations and three addition operations. Division by the DSP of the arctangent demodulator is implemented by multiplying the numerator by an inverse of the denominator. In order to determine the inverse of the denominator, the DSP utilizes a recursive routine which requires five iterations to produce accurate results. Each iteration of the recursive routine requires four multiplication operations and one addition operation. Lastly, differentiation by the DSP requires one addition operation.
From the foregoing, it can be seen that the DSP of the arctangent demodulator requires a total of twenty-six multiplication operations and nine addition operations in order to process each sample of the in-phase signal and quadrature signal in order to produce the voice/data signal.
Each computational operation executed by the DSP consumes electrical power. Since radios utilize portable rechargeable batteries to power the electronics thereof, it is desirable that such electronics use as little power as possible while providing their intended function in order to maximize the use of energy stored in the portable batteries.